feat: Inactive group headers for GroupMultiSelectPrompt

[AdrianGonz97]

Closes #197

By default, group headers are selectable, allowing you to quickly select all of the options within that group. Setting the new option, selectableGroups, to false will disable them, making the group headers inactive.

the following code snippet:

import { groupMultiselect } from "@clack/prompts";

await groupMultiselect({
	message: "Which libraries would you like to setup?",
	selectableGroups: false,
	cursorAt: "Bootstrap",
	options: {
		CSS: [
			{ label: "Bootstrap", value: "Bootstrap" },
			{ label: "TailwindCSS", value: "TailwindCSS" },
			{ label: "UnoCSS", value: "UnoCSS" },
			{ label: "Bulma", value: "Bulma" },
		],
		Testing: [
			{ label: "Playwright", value: "Playwright" },
			{ label: "Vitest", value: "Vitest" },
		],
		Database: [
			{ label: "Drizzle", value: "Drizzle" },
			{ label: "Prisma", value: "Prisma" },
		],
		Auth: [
			{ label: "Lucia", value: "Lucia" },
			{ label: "Auth.js", value: "Auth.js" },
		],
	},
});

will now produce this prompt: